This invention relates to a method of using an integrated, automatic analytical system for determining the distribution and the sizes of small particles present on the surface of certain polymer films, especially those used for special applications such as, for example, videotapes.
The polymer most commonly used for videotape film manufacture is poly(ethylene terephthalate). However, the technique of this invention is equally applicable to films of other materials, and the general principle on which this invention is based was originally developed for ordinary transparent packaging films, such as cellophane films, for determining their surface characteristics.
It is customary during the manufacture of poly(ethylene terephthalate) films to be used for making videotapes to add small particles to the film surface to reduce friction in windup. In order to obtain optimum surface properties, the height distribution of surface peaks (called asperities) must be carefully measured and controlled.
A paper by D. K. Owens presented at the International Microscopy symposium in Chicago in June 1960, and published in the "Proceedings" of that symposium, describes a technique for examining the surface of a transparent film known as metal shadowing. Briefly, the technique consists of metallizing the film surface by vacuum evaporation of the selected metal so that it travels from the vapor source to the film surface at an angle other than 90.degree.. Because of this angular path, the metal will not deposit behind asperities present on the film surface, creating clear areas known as "metallic shadows". The height of each asperity can be readily calculated from the length of the metallic shadow behind it and the angle of impact or "shadowing angle".
Such examinations were done in the past by photographing a selected metallized film surface with a camera attached to a light microscope, and then manually scanning the photographic plate, measuring the lengths of metallic shadows, and calculating the height distribution of the asperities. This operation was very slow and tedious and inherently error-prone.
There is, therefore, a need for improving the efficiency and accuracy of such metallized film surface examinations.